Tham khảo tài liệu 'electrical generation and distribution systems and power quality disturbances part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Power Quality and Voltage Sag Indices in Electrical Power Systems 147 severity. Observing the graph shown in Figure 4 there are 5 events per year where the voltage drops below 40 of nominal Voltage for s or longer. Equally there are 5 events per year where the voltage drops below 70 magnitude and 250 ms duration. Fig. 4. Voltage sag co-ordination chart The advantage of this method is that equipment behavior can be directly compared with system performance for a wide range of equipment. The disadvantage of the method is that a two-dimensional function is needed to describe the site. For comparison of different sites a smaller number of indices would be preferred. Calculation methods a. Method used by Detroit Edison The method calculates a sag score from the voltage magnitudes in the three phases Sabin 2000 . S 1 - V V V 6 This sag score is equal to the average voltage drop in the three phases. The larger the sag score the more severe the event is considered to be. b. Method proposed by Thallam A number of site indices can be calculated from the voltage sag energy Thallam 2000 . The Voltage Sag Energy Index VSEI is the sum of the voltage sag energies for all events measured at a given site during a given period VSEI E Evs _ i 7 i The Average Voltage Sag Energy Index AVSEI is the average of the voltage sag energies for all events measured at a given site during a given period 1 N AVSEI N E Evs _ i 8 148 Electrical Generation and Distribution Systems and Power Quality Disturbances A sensitive setting will result in a large number of shallow events with a low voltage sag energy and this in a lower value for AVSEI. The sag event frequency index at a particular location and period is suggested as the number of qualified sag events at a location and period Thallam Koellner 2003 . The System sag count index is the total number of qualified voltage sag events over the number of monitor locations. By the expression qualifying events it implies a voltage less than 90
